Emil, 9.5m should be perfect on the 127. I'll be running a 9.3 on this board on the PWA this year (fairly similar sizing). I even tried it with a 9.9m on the weekend and it's pretty nice for lightwind cruising - a little sticky in the gybes but still great fun.
The R16 will be a much better upgrade over the SL1. Also the 46cm might be on the small size with +9m sails. The R16's have a wider profile through the middle of the fin I believe; also they are much more upright so generate a lot more lift to get the nose of the board lifting a little more which helps fly it in light winds. I'll be running 48-50cm R16's in this board I think.
If you don't want to shell out on a new fin, try cutting 1-2mm off the front/back of the fin head and rake your 46cm about 2-3cm more upright in your box (so the head of the fin sits inside the finbox 2mm at the front). This should give you a bit more lift ;-)
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